1. Field of the Invention
The present invention relates to an electrostatic capacitance type ultrasound element, and an ultrasound endoscope including the ultrasound element.
2. Description of the Related Art
An ultrasound diagnostic method by which an inside of a body is irradiated with ultrasound to image a state of the inside of the body from an echo signal for diagnosis comes into widespread use. One of the ultrasound diagnostic apparatuses for use in the ultrasound diagnostic method is an ultrasound endoscope (hereinafter called a “US endoscope”). In a US endoscope, an ultrasound transducer is placed at a distal end rigid portion of an insertion portion that is introduced into a body. An ultrasound transducer has a function of converting an electric signal into ultrasound, transmitting the ultrasound into a body, and receiving the ultrasound reflected at the inside of the body to convert the ultrasound into an electric signal.
So far, for ultrasound transducers, ceramics piezoelectric materials containing lead having a large environmental load, for example, PZT (lead zirconate titanate) has been mainly used. In contrast with this, Caronti et al. discloses an electrostatic capacitance ultrasound transducer (capacitive micromachined ultrasonic transducer; hereinafter called a “c-MUT”) that is produced with use of a MEMS (micro electro mechanical systems) technique, and does not contain lead in the material. A c-MUT has an ultrasound cell (hereinafter, called a “US cell”) in which an upper electrode section and a lower electrode section are disposed to face each other via a void portion (cavity), as a unit element. A plurality of US cells with respective electrode sections connected by wiring sections are arranged, and an ultrasound element (hereinafter, called a “US element”) is configured.
The US cell vibrates a membrane (a vibration portion) including the upper electrode section by an electrostatic force by applying a voltage to between the lower electrode section and the upper electrode section, and generates ultrasound. When ultrasound enters from an outside, a space between both the electrodes changes, and therefore, the ultrasound is converted into an electric signal from a change of an electrostatic capacitance.
The US cell has a structure in which a plurality of functional layers are stacked. Therefore, if the placement positions of patterns of upper and lower functional layers are misaligned, the characteristics are impaired. For example, if the lower electrode section and the upper electrode section are not placed in correct positions, the areas of the electrodes facing each other, in other words, effective electrode areas likely decrease.
Japanese Patent Application Laid-Open Publication No. 2007-301023 discloses a c-MUT in which the size of an upper electrode section is smaller than the size of a cavity. In the aforementioned c-MUT, the size of the upper electrode section is small, and therefore, the areas of the electrode sections facing each other are small, but even if the formation position of the upper electrode section is misaligned, the effective electrode areas do not change as long as the upper electrode section is located in a directly upper portion of the cavity.